Knowing whether there is a vehicle in a predetermined area is a common need in the management of car parks, railroad crossings, emergency exits, access control, security areas and other applications where decisions must be made based on the presence or absence of a vehicle in a particular area. In covered areas, with some form of row, the presence or a vehicle in a particular area may be ascertained by one or more sensors suspended from the roof or from a structure attached thereto and situated directly over the area it is wanted to control. A common way of doing this is by way of sensors based on some type of radiation, for example ultrasonic or infrared radiation, because the transit time between an emitter and a receiver depends on whether the radiation emitted by the emitter is reflected from the floor or from another closer object, as may be the roof of a vehicle. An alternative solution, not requiring a sensor for each specific area it is wanted to control, is to install one or more television cameras directed towards the area of interest. By way of image analysis algorithms, it is possible to ascertain the number of vehicles there are in an area covered by the cameras. This solution is technically more complex, since it is necessary to be able to detect vehicles which are partly or completely hidden behind others, and it requires having a support for mounting the camera or cameras. In a building, this support may be fixed to a wall or on the ceiling, but in roofless areas, it is necessary to install some kind of post.
A cheaper alternative to the cameras may be placing detectors in the flooring of each lot it is desired to control. For the detection, one or other of the methods based on the emission and reception of some type of radiation which are normally applied from the ceiling may be adapted. But it is simpler to base the detection on a passive system which captures the alteration that the presence of a vehicle produces in some prior condition of the surroundings. Furthermore, the energy consumption of the passive detectors will be lower because it is not necessary to create said preexisting condition, unlike the detectors based on the emission and reception of radiation (radiofrequency, optical, mechanical-ultrasonic) which have to generate the radiation they emit. In large parking areas, and in general where there is no infrastructure allowing for an immediate supply of electric power to each watched lot, the possibility of supplying each detector from batteries reduces the installation costs. In these cases, another important quality of the detector is that its energy consumption should be the lowest possible.
A physical magnitude that can be measured in the ground and which changes when a vehicle passes is the mechanical stress in an element over which the wheel passes. Thus, WO/1996/01461 of Antonio Hernando Grande et al. describes a device for detecting parked vehicles based on detecting the mechanical stress produced by the vehicle wheel on its passage over a wire having one end fixed in a metal support and the other end of which is fixed to a semicircular support where there is adhered a strip of amorphous magnetic material. This strip forms the nucleus of a winding collecting the electromotive force induced by the passage of the wheel over the wire. It will be appreciated that, like happens with the piezoelectric sensors, pneumatic tubes and inductive coils, which detect a short transitory change due to the passage of the vehicle, this sensor has to be continuously in the active state, with the consequent energy consumption, and cannot detect stationary vehicles. Furthermore, a vehicle which, for example, passes over the sensor when going towards another parking place will give a signal when passing over the sensor of each place, but will end up parked in a particular place, whereby the detection count is greater than the number of parked vehicles.
Another physical magnitude which is altered with the passage of a vehicle, and even with its mere presence, is the magnetic field, because the abundant ferromagnetic material in the vehicles causes a disturbance in the Earth's magnetic field which lasts while the vehicle is present. This disturbance may be detected, for example, by magnetoresistive sensors (based on the magnetoresistive, anisotropic-AMR or giant-GMR effect) and therefore this method has been much applied to detect the passage of vehicles and somewhat less for detecting their presence [see, for example, M. J. Caruso, L. S. Withanawasam, “Vehicle Detection and Compass Applications using AMR Magnetic Sensors” http://www.ssec.honeywell.com/magnetic/datasheets/amr.pdf; P. Ripka, “Magnetic sensors for traffic control,” Proceedings of the International Symposium on Measurement and Control in Robotics (ISMCR 99), Tokyo, vol. 10, pp. 241-246, 1999; G. Rouse, H. French, H. Sasaki “A solid-state vehicle detector for roadway applications,” IEEE Proceedings on Vehicle Navigation and Information Systems Conference, Seattle, USA, pp. 11-16, July 1995; S. V. Marshall, “Vehicle Detection Using a Magnetic Field Sensor,” IEEE Transactions on Vehicular Technology, vol. 27, no. 2, pp. 65-68, May 1978; R. Lao, D. Czajkowski, “Magnetoresistors for Automobile Detection and Traffic Control,” Sensors, pp. 70-73, April 1996]. To detect a vehicle, it is sufficient to measure the magnetic disturbance in the direction of the predominant component of the Earth's magnetic field, which changes at different points of the Earth, according to the latitude, although it is also possible to detect a vehicle by measuring in other directions.
One problem common to all these applications based on magnetoresistors is that the energy consumption of these sensors is relatively high because their electrical resistance is of the order of 1 kΩ to 5 kΩ, and therefore if they are powered continuously, they can drain a battery in a very short time. If there is more than one sensor, for example, in sensor nodes including a magnetic sensor for detecting vehicles and several specific sensors for ascertaining the environmental conditions (temperature, rain, ice, etc.) and the system has no wired connection, for example, as disclosed in US 20060202863 A1 of Robert Kavaler, the energy consumption problem is even more serious. Furthermore, the refinements in magnetoresistor-based magnetometers applied to vehicle detection use an analog-to-digital converter as means for obtaining a digital signal corresponding to the magnetic disturbance measured, and said converter demands conditioning of the output from the magnetic sensor, which is obtained by amplifiers, as described for example in U.S. Pat. No. 5,491,475 to Gordon F. Rouse and William M. Volna and U.S. Pat. No. 6,546,344 to James A. Rodrian and Donald R. Janke. These signal conditioning circuits with amplifiers, filters, etc. imply additional energy consumption.
One obvious system for reducing the energy consumption is to power the detector intermittently at predetermined time intervals. But if it is wanted to have a reliable presence or absence detection, said intervals must be sufficiently short, and consequently the energy saving will greatly depend on the occupation rate of the area. US 20020190856 of Charles Howard discloses a solution for avoiding the magnetic sensor from having to be in continuous operation. It consists of using a vibration sensor which detects the vibration in the ground caused by a vehicle and which activates the magnetic sensor when vibration occurs and deactivates it when there has been no vibration for a period of time. The efficacy of this solution depends on the one hand on the consumption of the vibration sensor, which must be continuously active if continuous detection is wanted, and on the knowledge of the vibration signal which may be considered to be an indicator of the presence of a vehicle in the area of interest. But, in any case, the vibration sensor cannot detect the presence of a motionless vehicle. The vibration sensors cited in US 20020190856 are an electret microphone EM9765-422, the consumption of which is 0.5 mA when powered at 4.5V and an accelerometer ADXL202, the typical consumption of which when powered at 5 V is 0.6 mA. To these consumption rates, it is necessary to add the consumption of the components required for conditioning the output signal of the vibration sensor used, of the controller to which its signals are communicated and of the power supply for the ensemble.
WO-20061063079A2 discloses a detector assembly to detect presence of a vehicle, including a microprocessor and a passive magnetic sensor which, in one embodiment, is coupled to an active sensor like ultra-sonic, infra-red, radar, laser, capacitive or photoelectric. The operating way of this detector assembly consists in that the microcontroller periodically activates the magnetic sensor to detect the presence of a vehicle and, when the vehicle has been detected by said magnetic sensor, the active sensor is activated to confirm this presence. The position from where the detection is carried out is not specified.
The analysis of the information available to date shows that there is no method for continuously and reliably detecting the presence of a vehicle in a particular area and which, in the absence of vehicles has an energy consumption of less than 30 microwatts (10 microamperes with a supply of 3 V). We have found that by placing an optical sensor in the flooring which when detecting a low level of illumination starts up a magnetic sensor in the same point, to check whether the reduced level of illumination is due to a magnetic disturbance, allows a continuous reliable detection of the presence of vehicles, with an energy consumption much lower than that of a continuously powered magnetic sensor, or that of a magnetic sensor activated when a vibration sensor detects a signal indicating the approach of a vehicle.